An important feature of golf cart batteries is the number of

An important feature of golf cart batteries is the number of minutes they will perform before needing to be recharged. A certain manufacturer adver- tises batteries that will run, under a 75-amp discharge test, for an average of 125 minutes, with a standard deviation of 5 minutes.

a) Find an interval that contains at least 20% of the performance periods for batteries of this type.

b) Would you expect many batteries to die out in less than 100 minutes? Why?

i got part a .. the interval is ( 109.2,140.8)

but how can i get part b (explain please )

Solution

The answer is no because 3 standard deviations below is 125 - 3(5) = 110. Anything less than that is absolutely too rare with chance of it happening close to 0%.